Groundwater Flow Lab
February 24th, 2014
ACF/Class
Background: Groundwater is the source of baseflow to creeks and rivers. For Hatchet Creek,
the aquifer that supplies baseflow is the surficial aquifer, which is perched on top of the
Hawthorn. In other words, at baseflow, the flow in the creek seeps from the banks and stream
bed along the length of the channel. In this way, flow can be maintained (and accumulate along
the length of the stream) even where no surface tributary is present. The Santa Fe and
Suwannee Rivers are fed in the same way in their lower reaches, but in those cases by the
Floridan Aquifer (via springs, seeps and diffuse groundwater flow). Regardless of the aquifer
source, it is essential to understand the rate of water delivery at baseflow because of the State
mandate to maintain and protect minimum flows and levels (MFLs). In order to determine the
rate of baseflow seepage to a creek, we need to know several things, including properties of
the media (the sand through which the water flows) and the hydraulic gradient. This lab would
ordinarily measure both, but the regional drought conditions mean that no baseflow is present
in Hatchet Creek. Despite being a poor surrogate for making the measurements yourselves,
this year we will be evaluating these properties using previously collected data.
In this lab, we estimate the hydraulic gradients that drive water into (or out of) a river from the
surficial aquifer. The main ingredients of that flow rate are the slope of water table (the
gradient), the length of the flow path, the cross-sectional area of the flow path, and the
saturated hydraulic conductivity (Ksat), which describes the rate at which water flows through
the particular sediments being studied. Ksat can be estimated from measures of texture and
organic matter content, or it can be measured directly; we’ll do the latter using a slug test.
A slug test is a type of aquifer test where water is added or removed from a groundwater well,
and the change in hydraulic head is monitored through time to determine the subsurface
hydraulic characteristics. It is a method used by hydrogeologists and civil engineers to
determine the transmissivity of the aquifer media.
Once we know Ksat we can estimate the slope of the potentiometric surface, and the length of
the flow path; four wells have been installed along a seepage face into Hatchet Creek, and
these will be the particular measurements that you’ll use.
Objective: To measure the rate of lateral seepage into/out of Hatchet Creek (floodplain).
Procedure:
1. We will measure groundwater flow at four locations (one for each group).
2. At each site there will be 4 measurements (refer to Fig. 1):
a. Distance between well site and stream (in cm)
b. Height of the stream (in cm; note that values will be negative) below
the laser level (fixed for all sites)
c. Height of the well casing (in cm; note that values will be negative)
below the laser level
d. Height of the water (in cm; again values will be negative) below the
well casing.
3. Estimate the hydraulic gradient (slope of the water table). This is the change in
water height (ΔH) over some distance (ΔL).
4. Estimate the cross sectional area of flow. We will assume that it’s the height of
the water table above the stream, and we’re interested in the flow for a given
meter of stream length. As such, the cross-sectional area is just the hydraulic
gradient (ΔH) multiplied by 100 to get an area in cm2.
5. Estimate the hydraulic conductivity of the sediments by using the slug test data.
These data were obtained using a pressure transducer placed in the well. After
some short period of equilibration, we added a “slug” of water (hence the
name), and watched as the water level recovered to its previous value. This
process is fast when the hydraulic conductivity is high (e.g., in sand or gravel)
and slow when Ksat is low (clay or rock).
Fig. 1 – Schematic of wells and distance measurements required.
Equations: We will be implementing the Hvorslev method to extract Ksat (saturated
hydraulic conductivity) from the rate of well level decline. This method relies on the
relationship between time (in minutes) and relative head level (Ln[h/h 0], where h is the
head at time t and h0 is the head at the start of the slug test. It also depends on the
geometry of the well. The equation that you’ll use is:
r 2 Ln( Le / R)
K
2 LeT37
Where K is the hydraulic conductivity (units of length per time, where length is defined
based on the units of r, R and Le, and time is the units of T37), r is the radius of the well
casing (2.5 cm), R is the radius of the well screen (which is the same as r in our case), Le
is the length of the well screen (45 cm) and T37 is the time required (in seconds) to reach
37% of slug head remaining (h/h0 = 0.37). If you don’t get to 37% of head loss on the
time during which the transducer was in the well, you will need to use the graphed
relationship between time and h/h0 to estimate this time.
Darcy’s Law
Darcy’s Law predicts the flow of water through a groundwater media. It is based on the
measurements that you’ve taken in the field (distances and elevation differences). To
compute the flow (Q = m3/day; note that most of your measurements are in cm, and
per second…be careful), you will need to use Darcy’s Law:
Q = K * A * Δh/L
where Q is flow (in m3/day), K is the hydraulic conductivity (in m/day), A is the crosssectional area (in m2 - you’ll assume a width and depth of 1 m), Δh is the potentiometric
gradient (in m) and L is the distance (also in m).
Fig. 2 – Schematic of slug test.
Fig. 3 – Time vs. normalized head for a typical well slug test [note: log-scale on y-axis]
Report:
1. Report the Ksat for each well from the slug test data. Show a plot of time vs.
normalized head (like Fig. 3). Compare Ksat values among wells. Are they the
same?
2. Compute the potentiometric gradient for each well (in meters) from the elevation
data provided.
3. Compute the flow per unit streamlength (use 1 meter as the unit length) towards
(or away from) the creek for each well pair using the Ksat values from each group,
the gradient computed in #2 and the distance between the wells. Assume that
the cross-sectional area is defined by the 1 m of stream length and the height of
the water table above the stream at each well location.
4. Assume that the measurements of flow toward the creek adequately represent
the contribution for the entire length of creek as it passes through Austin Cary
Forest. That distance is 1200 m. How much water would you estimate seeps
from Austin Cary to Hatchet Creek at baseflow? Remember, ACMF only
contributes flow from the north side of the creek. Assume Hatchet Creek base
flow is 0.1 m3 per second.
5. How much length of creek (both sides) would be necessary to create that
baseflow (0.1 m3/s) if the bank seepage rate were the same as the average of our
measurements?